When people say “they're engaged,” the phrase “to be married” 'is understood'. [=“they're engaged” is understood to mean “they're engaged to be married”]
We still don't fully 'understand' the causes of the disease. = The causes of the disease are still not fully 'understood'.
He believed it 'was understood' [=believed that everyone had agreed] that he would be paid for the work.
(..) I would have thought we were perfectly clear the first time. - You're not welcome. - You once spoke of your respect for our father because he understood the necessity of working with one's rivals. My dear, you have been stripped of your dignity and authority, publically shamed, and (..)2016 TvShow: Game of Thrones Title: Book of the Stranger Season: 6 Episode 4